1. Field of the Invention
The present invention relates to microwave down converters and interdigital filters and, more particularly, to the combining of one or more interdigital filters into the down converter and the method for constructing the integrated combination.
2. Discussion of Prior Art
Various types of down converters are readily available for converting microwave signals such as in the range of frequencies from 2150 to 2162 MHz (MDS) and 2500 to 2686 MHz (ITFS) received by an appropriate antenna such as an MDS antenna to a corresponding electrical signal for delivery to a receiver. A conventional down converter for an MDS antenna is of the type manufactured by Conifer Corporation, P.0. Box 1025, Burlington, Iowa 52601, available as the QL Series.
Conventional down converters generally use two types of integrated filters. The first type of integrated band pass filter is a printed filter and the second type is a dual cavity filter. While such filters function adequately for single channel MDS applications, they do not function adequately in multiple channel situations.
Interdigital filters exhibiting greater IF rejection, better out-of-band rejection and lower in-band insertion loss than printed and dual cavity filters are commercially available for use with down converters. The theoretical basis for the design of interdigital filters is well known as set forth in the article by Jerry Hinshaw and Shahrokh Monemzadeh, "Computer-Aided Interdigital Bandpass Filter Design." Ham Radio Magazine, January 1985, Pages 12-26. Such interdigital filters, however, are available only as separate circuits and, therefore, require a separate housing and a separate jumper cable to interconnect the interdigital filter with the down converter. Both the interdigital and the down converter are placed near the antenna in the outside environment thereby necessitating a waterproof housing for each. The provision of a separate waterproof housing for the interdigital filter is expensive. Likewise, the provision of a jumper cable not only adds to the cost, but also degrades the signal as well as provide an impedance mismatching problem.
Furthermore, a conventional interdigital filter such as those available from Microwave Filter Company, Inc., 6743 Kinne St., East Syracuse, N.Y. 13057 as Model No. 3746 are expensive to manufacture generally requiring machined components. For example, the interdigital filter Model No. 3746 for MDS applications from Microwave Filter is commercially available at prices from $95 to $135. The Conifer Down Converter is also commercially available at about a price of $80. The combined price with a $10 jumper cable would be about $185 for the two separate units. Under the teachings of the present invention, the integration of the interdigital filter into the down converter would result in a price of about $100.
Even more dramatic cost savings result if multiple interdigital filters are used in parallel to filter the input signal of the down converter. This permits a number of separate microwave frequency bands received by an appropriate antenna to be filtered and converted to corresponding predetermined output frequency bands by means of a single down converter. One interdigital filter is used for each input frequency band. For example, by using two interdigital filters, both the MDS and ITFS bands can be converted by a single down converter.